An optical navigation device, such as an optical mouse, typically includes a light source to illuminate a navigation surface and an optical navigation sensor integrated circuit (IC) that functions as a miniature digital camera to continually collect images of the illuminated navigation surface and to determine the speed and direction that the device is being moved across the surface by comparing sequentially recorded frames of image information. Image frames are collected at a very high rate and the resolution of the optical navigation sensor IC is high enough to detect very small movements of the device relative to the navigation surface.
When an optical mouse is used on most opaque surfaces, such as desktops, the collected image frames have enough features for the optical navigation sensor IC to determine relative movement between image frames. However, using an optical mouse on a transparent surface, such as a plate of glass that sits on a desktop, presents unique challenges. In particular, the top surface of the glass is usually too smooth to provide distinguishable features in the collected image frames and the thickness of the glass changes the geometry between the light source, the optical navigation sensor IC, and the underlying desktop such that an insufficient amount of light is reflected from the desktop to the optical navigation sensor IC.
An optical mouse could be designed for dedicated use on a desktop that is covered by a glass plate of known thickness. Although such an application-specific design is possible, it is not practical because an optical mouse may be used on both transparent and opaque surfaces over its lifetime and because the thickness of glass plates that cover desktops is difficult to predict and may change from plate to plate.